Ore dressing



Jan. 55 1943. RL. PATTERSON 2,307,064

oas DRESSING Filed July 27, 1940 /fdfaew y yV ATTORNEYS Patented Jan. 5, 1943 by mesne assignments, to Powder Metals and Alloys, Inc., New York, N. Y., a corporation of Delaware Application July zu, 1940, serial No. 348,002

(ci. :z3-s4) 3 Claims.

powder metallurgical methods. Finely-divided iron oxide can be reduced'directly to iron powder relatively inexpensively by so-called sponge iron processes employing gaseous reducing agents but without substantial fusion. However, in order to obtain iron powder of high purity by such methods the iron oxide treated should be relatively free from gangue. In many instances, heretofore customary orer dressing processes do not produce iron oxide concentrate that is suitable for the production of commercially acceptable iron powder. through direct reduction as described above. Thus, iron oxide concentrates produced by treating the ground ore on magnetic separators contain excessive proportions of gangue material, principally siliceous.

As a result of my investigations, I have discovered that gangue material which resists removal from oxide minerals and ores of iron, chromium and manganese by conventional processes (such as fine grinding followed by magnetic separation of the oxide) may be removed effectively and to an adequate degree by subjecting the mineral in relatively coarse state and substantially free from fines to a pneumatic tabling operation. I have found that with most oxide ores of iron, chromium and manganese, and particularly with magnetite ores, relatively coarse crushing is adequate to sever the gangue and oxide suiiiciently to permit the production of a very high grade iron oxide concentration, provided that the concentration is carried out by pneumatic tabling in the absence of ilnes. Thus, in the process of my invention fine crushing,

dry condition to pneumatic tabling to remove the gangue.

The size of product to be subjected to' pneumatic tabling will vary somewhat with the nature of the ore but, in general, the product treated should be minus about four to eight mesh and plus about sixty-five mesh. Moreover, im-

proved results are obtained if the product to be I subjected to pneumatic tabling is rst divided into several sized fractions of different degrees of coarseness, each of which is tabled separately. Generally speaking, th'e greater the number of sized fractions into which the product is divided, the higher will be the grade of the concentrate produced. Thus, in an operation in which a feed ranging in size from eight to sixty- .live mesh is separated into eight sized fractions which are tabled separately, the concentrate contains substantially less silica than in cases where the same feed is tabled without separation, or an operation in which the feed is divided into, say, two sized fractions that are tabled separately. The optimum number of fractions for a given ore will depend upon commercial considerations but, in general, it is advisable to separate the feed into ai; least three sized fractions, for example, one, minus eight mesh plus twenty mesh, the second, minus twenty mesh plus thirty-flve mesh, and a third, minus thirty-live mesh plus sixty-five mesh.

As indicated above, the sized fractions are subjected to separate pneumatic tabling operations, a concentrate of high purity and a tailing being produced in each instance. In general, the coarser ,fractions produce concentrates of higher grade .but at the same time higher proportions of tailings.

The tailings, which contain most of the gangue, are removed from the system.

The concentrates preferably are combined into a single mass which is crushed to pass mesh. 'I'he crushed concentrate (after a de-dusting treatment to remove al small proportion of very line particles, say minus 400 mesh) may be converted directly into high grade metal powder by \reduction with a suitable reducing gas, say hyldrogen, without substantial fusion. Preferably, `the reduction of the ground concentrate is conducted according to a process described and claimed in the copending application of Charles Hardy, Serial No. 310,791, filed December 23, i939. Such a process involves reduction of the oxide without substantial fusion employing a deck, impurities that have not been removed in the prior pneumatic tabling operation being separated during reduction. fV

In the treatment of oxide minerals of metal selected from the group consisting of iron, chromium and manganese to remove gangue therefrom and produce a high grade concentrate suitable for production of a metal powder substantially free from gangue by direct reduction of the oxide, my invention contemplates the improvement which comprises subjecting the mineral to a preliminary concentration operation to produce a relatively coarse but high grade concentrate containing only a small proportion of gangue, separating the resulting concentrate into a plurality of sized fractions of different degrees of coarseness, but all of which are substantially free of particles coarser than 4 mesh and particles ner than 65 mesh, subjecting the several fractions separately to pneumatic tabling to produce a plurality of concentrates low in gangue, and a plurality of tailings, crushing the concentrates to pass 100 mesh, and de-dusting the crushed concentrates thereby removing additional gangue.

The process of my invention is applicable to a great variety of iron oxide ores vand concentrates and will serve to remove gangue from hematite, limonite, magnetite and other iron oxide minerals, as well as from chromite and oxidized manganese ores, su'ch as those in which the manganese occurs as pyrolusite, psilomelane, braunite and manganite. The process, however, is particularly desirable for treatment of magnetite ores or concentrates, and it is described in detail hereinafter as applied to the treatment of a relatively high grade magnetite material that has been subjected to preliminary, concentration by known methods to remove large` particles of gangue. The detailed description should be taken in conjunction with the flow sheet.

In the process illustrated, the raw material is a rough magnetite concentrate produced from ore by a process involving heating the ore with carbon and magnetic separation. A ,"typic'al analysis of the rough concentrate fed to the process is as follows:

I Per cent by wt. Per cent by Wt. Fe 67.00 as Fe304 92.5

The feed, as delivered to the concentrating system illustrated on the flow sheet, is crushed to pass eight mesh and is subjected to a dry screening operation that produces four (4) products, viz:

Screen size: Per cent by Weight of total -8 mesh +20 mesh 28.8 20 mesh +35 mesh 33.6 4-35 mesh +65 mesh 27.3 -65 mesh 10.3

Total 100.0

A variety of forms of screens or other sizing apparatus may be employed to separate the feed into the above-noted fractions. However, I prefer to employ a series of superposed screens that are set at a predetermined angle and are supported by spring steel strips from a solidly built base. These'screens are vibrated in a horizontal plane by a reciprocating drive. In the instant case, the screening apparatus comprises a twenty mesh screen I0, a thirty-five mesh screen II and a sixty-five mesh screen I2. The oversizes from the respective screens are the three sized fractions I3, I4, I5 to be subjected separately to pneumatic tabling. The undersize I1 of the sixty-five mesh screen consists of fines which may be discarded or treated separately for the manufacture of metallic iron or ceramics. The minus sixty-five mesh material II is rela tively high in silica (2.97-3.14%).

The respective sized fractions I3, I4, I5 are sent to separate pneumatic concentrating tables I 8, I9, 20. Several forms of commercially available pneumatic concentrating tables are suitable for the practice of the invention. However, I prefer to employ a pneumatic table known as an Air-float-separator and sold by Sutton, Steele and Steele, Inc. of Dallas-Texas. 'Ihe Sutton- Steele table resembles in size and shape an ordinary wet concentrating table in that it has an oscillating deck inclined longitudinally and laterally and equipped with riiiles. The deck of the table is formed of pervious cloth or screen and air is passed upwardly through the deck under pressure from a fan provided with shutters for controlling the amount of air. Underlying the deck is an air chamber provided with means for distributing and regulating the air pressure.

Both deck and air chamber are mounted on a running gear which is given a differential stroke by means of eccentrics. Both the longitudinal and lateral slope of the deck surface and the speed and amplitude of deck oscillations may be regulated.

Another and older form of the Sutton-Steele pneumatic table is illustrated and described on pages 573-4 of the Textbook of Ore Dressing by Truscott (MacMillan, 1923). In this older form of table, there are no riilles and the air chamber and deck are supported on springs and shaken by a cam mechanism.

Other forms of pneumatic shaking tables may be employed, for example, a table developed for treating bituminous coal and illustrated on page 938 of Handbook of Ore Dressing by Taggart (Wiley, 1927).

In the instant case, each sized fraction of magnetite is fed separately to an upper corner of a table. Air passing upwardly through the porous deck acts as a pneumatic cushion upon which the particles rest. Oscillation of the table causes the materialthereon to become stratified with the heavier particles nearer the'deck and with the light particles of gangue above. The gangue is shaken oil. the side of the table and the magnetite concentrate passes to the end of the table to a collecting chute.

To consider the tables separately, the table I8 receives the coarsest fraction I3 -eight mesh+twenty mesh) which amounts to 28.8% of the total feed. The table I8 produces a concentrate amounting to 92.3% by weight of the coarsel fraction with 7.7% tails. The table I9 receives the intermediate fraction I4 (-twenty mesh+thirtyve mesh) which amounts to 33.6% of the total feed. This table produces a concentrate which amounts to 96.9% of the intermediate fraction with 3.1% tails. The least coarse fraction l is fed to the table 20. The fraction I5 represents 27.3% of the original feed and is minus thirty-five plus sixty-live mesh in size. It is divided on the table into concentrates representing 98.5% by weight concentrates and the balance of 1.5% tailings. Y

Under reasonably satisfactory conditions of op-` eration, the three concentrates are all high grade and considerably purer than concentrates produced from the feed by heretofore customary methods. Thus, the best concentrate produced heretofore from the feed material by.heretofore customary methods contained not less than .6% SiOz. In contrast, the concentrates 20A, 2|, 22 produced, respectively, on tables I8, I9 and 20 contained the following proportions of silica:

Per cent Concentrate: I SiOz 20A (minus 8 mesh plus 20 mesh) .177 2l (minus 20 mesh plus 3'5 mesh) .186 22 (minus .35 mesh plus 65 mesh) .203

Although all three of the concentrates are very low in silica, the coarsest of the concentrates is the best in this respect.

If, instead of separating the minus eight mesh plus sixty-five mesh material into three fractions, the entire mass is treated on a single table a relatively high grade concentrate (38% SiO2) is produced. On the other hand, if the material is divided into, say, eight to ten sized fractions by employing eight screen gradations instead of three, a marked improvement in purity of concentrate is obtained with an average silica content of only .O5 to .1%. Consequently, if almost complete elmination of gangue is required the number of parallel table stages should be substantially increased over the' three shown on the flow sheet. However, the average grade of concentrate produced with three parallel table stages is satisfactory for producing high grade iron powder according to the process described and claimed in the aforementioned copending application of Charles Hardy. n

As shown on the flow' sheet, the three concentrates from the three parallel stages are combined in a storage bin 23 and the combined concen'trates 26 is sent to a ball mill 25 operated in closed circuit with a cyclone separator 26 and the discharge end of the mill through a conduit 28 to the separator. Material 29 over 100 mesh in size is removed in the cyclone separator and returned to the feed end of the mill. Ground concentrate 30 minus 100 mesh in size is discharged from the cyclone separator.

During crushing, a small proportion of the concentrate is comminuted to an almost impalpable powder, and this dust (which, in general, is minus 400 mesh in size and constitutes about 2% of the concentrate) should be removed before the concentrate is subjected to direct reduction. rIjhe dust flies when the concentrate is subjected to reduction with hydrogen on a shaking table disposed in a heating chamber. The flying dust tends to escape from the chamber and to form metallic accretions within it. Moreover, the dust appears to interfere, to some extent at least, with the reduction of the larger particles on the table.

Lastly, the dust is high in silica and other gangue ingredients, and its removal raises the grade of the concentrate appreciably.

As shown in the ilow sheet, de-dusting of the ground concentrate may be accomplished conveniently in a pneumatic separator or deduster 3|, which has a hollow shaft down which the concentrate falls to the center of a rotating impeller. The impeller throws the concentrate outwardly, and the coarse particles thereof impinge against a cylindrical. baille from which they fall into a hopper and are withdrawn as de-dusted concentrate 32. Dust 33 due to its high surface per unit of mass, is entrained'in the air and is withdrawn in suspension over the top of the baille and removed from the system.

The de-dusted concentrate is in condition to be reduced directly to iron powder, preferably by tabling in the presence of a hot reducing gas, such as hydrogen. The preferred apparatus for the oxide reduction 34 comprises a shaking table ofl heat resistant material mounted in a heating chamber and having a porous deck through which hot hydrogen is forced into a bed on concentrates on .the table. Concentration and reduction take pla-ce simultaneously on the table. Incompletely reduced particles, due to their lower density, are retained on the table for a longer period of time and are either completely reduced or separated from particles which have been so reduced. Details of such apparatus and its mode of operation are disclosed in the copending application Serial No. 310,791, filed December 23, 1939, by Charles Hardy.

I claim:

l. In the treatment of oxide minerals of metals selected from the group consisting of iron, chromium and manganese to remove gangue therefrom and produce a highY grade concentrate suitable for production of a metal powder substan` tially free from gangue by direct reduction of the oxide, the improvement which comprises subjecting the mineral to a preliminary concentration operation to produce a relatively' coarse but high grade concentrate containing only a relatively small proportion of gangue, separating the resulting concentrate into aplurality of sized 'fractions of different degrees of coarseness but all of which are substantially free of particles coarser than four meshV and particles finer than sixty-five mesh, subjectingl the several fractions separately to pneumatic tabling to produce a plurality of concentrates low in gangue and a plurality of tailings, crushing the concentrates to pass mesh, and de-dusting the crushed concentrates, thereby removing additional gangue.

2. In the treatment of iron oxide minerals to remove gangue therefrom and produce a high grade concentrate suitable for production of an' iron powder substantially free from gangue by direct reduction of the concentrate, theimprovement which comprises subjecting the iron oxide mineral to a preliminary concentration operation to produce a relatively coarse but high grade concentrate containing only a relatively small proportion of gangue, separating the resulting concentrate into at least three sized fractions of different degrees of coarseness but all of which are substantially free of particles coarser than four mesh and particles finer than sixty-five mesh, subjecting the several fractions separately to pneumatic tabling to produce a plurality of concentrates low in gangue and a plurality of tailings, crushing the concentrates to pass 100 mesh, and de-dusting the crushed concentrates, thus removing additional gangue.

3. In the 'treatment of magnetite ore to remove gangue therefrom and produce a high grade concentrate suitable 4for production of iron powder substantially free from gangue by direct reduction of the magnetite, the improvement which comprises subjecting the magnetite to a preliminary concentration operation involving heating it with carbon and subjecting it 5 to magnetic separation, thereby producing a relative coarse butl high grade concentrate containing only a relatively small proportion of gangue, separating the resulting concentrate into a plurality of sized fractions of different 10 

